The invention relates to an apparatus for accelerating and transporting a beam of charged particles. In particular, it relates to an electron linear accelerator (LINAC) used in radiotherapy.
A typical LINAC contains a vertical stand and a gantry supported by the stand so that it can rotate around a horizontal axis ("isocenter"). The gantry has two horizontally extended arms one of which houses an electron source, an accelerating structure and, at its outer end, a beam bending magnet system and a so called "lower treatment head" encasing various elements for transforming the bent beam into the final treatment beam. The treatment beam is then projected through the isocenter to a beam detector mounted at the outer end of the other arm which also carries a counter weight for balancing the gantry bearing.
The bending system, which typically consists of a vacuum-tight beam envelope surrounded by a number of magnet poles, coils and flux return elements, should be designed so that it not only translates incoming particles with different energies and trajectories into a tight parallel beam, but also affords a low isocenter and reduced torques at the gantry bearing.
To obtain good electro-optical qualities, magnet systems have been developed which deflect the electron beam by 270.degree.. These systems, examples of which are described in Medical Physics 11 (1984) pages 105 to 128, in particular section V, D., have reasonably low isocenters but are relatively heavy and bulky. Moreover, they are mechanically complicated because special steps must be taken to maintain an extremely high vacuum of about 10.sup.-9 torr in the envelope and to avoid thermal stresses in the exit window (see, for instance U.S. Pat. No. 4,293,772).
It is therefore an object of this invention to provide a charged particle beam accelerating and transporting apparatus having acceptable beam transportation characteristics and being nonetheless small and light.
It is another object of this invention to provide a charged particle beam accelerating and transporting apparatus which makes it possible for a LINAC to have a lower isocenter.
It is a further object of this invention to provide a charged particle beam accelerating and transporting apparatus which is of a simple construction and, in particular, does not require special means to prevent the beam windows from being overheated.
It is yet another object of this invention to improve on existing apparatus for accelerating and transporting a charged particle beam.